1. Field of the Invention
The present invention relates to improvements in a vertical axis translation mechanism for driving a table in a vertical direction.
2. Description of the Related Art
With the increasing precision and increasingly high level of integration in optical devices, electronic devices, and the like, in recent years, precision in the order of nanometres is required in the components constituting such devices. Therefore, accuracy of extremely high resolution is required in the machine tools, steppers, electron beam drawing devices, and the like, used to process these high-precision components. Generally, it is common for such positioning devices to be controlled by CNC-controlled rotating servo motors or linear motors, and in many machine tools, known as ultra high precision processing machines, for processing the aforementioned high-precision components, high-resolution servo motors are used, in particular, and furthermore, air bearings are used for the guide surfaces.
Generally, in a machine tool, such as a machining device, a tool is attached to a spindle, and work carried on a table is cut by causing the tool to rotate. In this case, in order to perform infeed cutting into the work, a vertical axle is required in order to move the table linearly in a vertical direction to the table. Usually, a spindle head is caused to move vertically by attaching a movable slide to a guide fixed to the column, or the like. In this structure, the weight of the spindle, the motor and other components for driving this spindle is supported by the column itself, and since the column has a cantilevered structure, flexing of the column is liable to occur, thereby leading to a decline in manufacturing precision. For this reason, such a mechanism is not suitable for ultra high precision machines for processing ultra high precision components.
Therefore, in an ultra high precision processing machine, the spindle to which the tool is attached is fixed and is not capable of vertical movement, and a vertical axis translation mechanism is provided for performing infeed cutting operations by causing the table carrying the work to move in a vertical direction. For example, Japanese Patent Application Laid-open No. 86026/1998 discloses a vertical axis translation mechanism for causing a table to move in a vertical direction, directly by means of a driving motor, via a large-diameter feed screw.
This vertical axis translation mechanism has a merit in that the weight on the table is transferred directly to the full circumference of the feed screw, and hence the mechanism does not assume a cantilever state and it is capable of extremely fine positioning without receiving any offset weight effects. However, this mechanism does involve a problem in that the table surface may slope, depending on the movement position of the table, and be impossible to maintain in a horizontal attitude, due to the manufacturing accuracy of the screw and nut in the feed screw mechanism. In other words, it generates a yawing or pitching effect as seen generally in feeder mechanisms, whereby the table surface oscillates as the table moves vertically. Therefore, the table is held in an inclined state when it is positioned, causing processing accuracy to fall.
Generally, in a machine tool, the straightness of the movable slide has a great effect on the accuracy of the shape of the machined item. In a slide for a machine tool having a feed screw mechanism, the machining precision of the feed screw and nut as well as the accuracy of the shape of the guide surface influence the accuracy of movement of the slide, and these factors, drawing together, affect the straightness of the machine tool. In an ultra high precision machine tool as described above, this effect is particularly notable, and since the straightness of the machine tool is reflected directly in the machining precision when machining items requiring a very high degree of finishing precision, yawing and pitching of the kind described above occurs due to the machining precision of the feed screw and nut, with the result that the table cannot be held in a horizontal attitude when it is positioned, and hence the precision of the machined shape is liable to decline.
In an ultra high precision machine tool, an air bearing is often employed in order to raise machining precision. The invention disclosed in the aforementioned Japanese Patent Application Laid-open No. 86026/1998 is a vertical axis translation mechanism which uses a static pressure air screw type translation motor based on a large diameter screw. In a feeder mechanism using a static pressure air screw, slight oscillations occur in the nut as it moves along the screw, due to manufacturing errors in the screw and nut, unevenness in the air bearing gap, and the like, and hence even in a vertical axis translation mechanism using a static pressure air screw of this kind, the table surface may be inclined when the table is positioned, hence causing machining accuracy to decline.
Moreover, in a machine tool using a feed screw mechanism, there are errors in the pitch between each revolution of the screw. In an ultra high precision machine tool having a static pressure air screw, as described above, such errors are slight, but they still exist. Conventionally, a method is adopted whereby previously measured pitch errors are stored as pitch error compensating data in a memory inside the control device, and the pitch error compensation is performed accordingly during the positional control process. However, in the aforementioned vertical axis translation mechanisms, the table may be inclined, and therefore it is very difficult to apply this compensating method.